Shock-absorbing struts are commonly used in vehicle suspension systems for absorbing and damping transient loads imposed on the suspension system. Two types of struts are commonly used. So-called "oleo" struts have an internal oil-filled cavity and an orifice through which oil flows upon compression of the strut, such that motion of the piston is damped by the flow of viscous oil through the orifice. The other type of strut in common use is the "oleo-pneumatic" strut which acts as both a damper and a spring by means of an oil-filled cavity and a compressible gas-filled chamber. In both types of devices, the degree of damping is chosen so as to effectively manage the types of transient loads expected to be imposed on the device during use.
Shock struts for use in aircraft must be able to absorb and damp loads imposed during a number of diverse use regimes, including landing, taxi, and takeoff. Unfortunately, a damping rate which may be suitable for absorbing the types of loads experienced during landing may not be suitable for damping the types of loads experienced during taxi. Thus, the design of the strut is necessarily compromised between the disparate requirements of the landing and taxi modes. Consequently, the rebound damping characteristics of the strut may be less than ideal during landing.
Aircraft rebound occurs when the aircraft, after the landing gear shock struts have been compressed by the forces of initial touchdown on the runway, is propelled back upward by the spring action of the landing gear shock struts. If shock strut rebound occurs too rapidly and forcefully, the aircraft may be propelled upward with enough energy to cause it to leave the runway. This is undesirable because during the rebound period, braking efficiency is reduced, resulting in a greater runway length required to bring the aircraft to a stop. Furthermore, should the wing lift spoilers extend on initial touchdown, and the aircraft then rebound with the spoilers still extended, subsequent ground loads may be unnecessarily severe as the aircraft touches down a second time.
Accordingly, aircraft shock struts are sometimes designed to prevent substantial rebound by building a high degree of damping into the strut. Such "stiff" shock struts, however, tend to transmit much of the transient loads imposed on the landing gear by surface discontinuities such as bumps on the ground during taxiing of the aircraft, resulting in passenger discomfort and possible structural damage to the strut if the piston bottoms in the cylinder.
As a partial solution to some of the problems described above, dual-rate shock struts have been developed which have two different damping rates depending on how hard the strut is compressed. For example, shock struts having overpressure relief valves are known, an example being described in U.S. Pat. No. 3,598,207 issued to Hartel. The aircraft strut described in the Hartel patent includes an oil bypass valve between the high-pressure oil chamber and the low-pressure oil chamber of the strut. The bypass valve is held closed by compression springs in a relaxed state of the strut and during initial compression of the strut, such as at initial landing touchdown, and accordingly the damping rate of the strut is high. Furthermore, the strut is configured to maintain the bypass valve closed until the aircraft has stopped its vertical sink velocity and the strut piston movement has ceased, thereby equalizing the oil pressures in the two chambers. However, the bypass valve is designed to open when a predetermined increased pressure exists in both the high-pressure and low-pressure oil chambers, such as during the last portion of piston stroke during landing as well as during taxiing of the aircraft. Accordingly, the damping rate of the strut is reduced during taxi relative to the damping rate during landing. The Hartel strut, however, does not solve the strut rebound problem because the bypass valve is open as the piston begins to be extended following the initial compression stroke, and remains open until the pressure in the oil chambers falls below the predetermined value such that the compression springs can overcome the fluid pressure forces acting on the bypass valve plunger and close the valve. Thus, the strut would have an undesirable rebound tendency.